This invention relates to clock recovery systems for generating a clock signal synchronous with a received data stream.
It is now commonplace to transmit digital data over a radio frequency channel. In order to interpret the received data, it is necessary to make a decision as to whether each particular data "cell" is a "1" or a "0". Thus, it is necessary to identify when bit cells occur. To do this, it is necessary to recover or reconstruct the clock signal from which the data was originally generated. A variety of arrangements for clock recovery are known.
Most digital clock recovery systems use arrangements such as discussed in Data Transmission by W. R. Bennett and J. R. Davey (see pages 260-262). In the common system discussed in that publication, the transitions of the data are used to define edges. These edges are defined as either being "early" or "late" with respect to a local clock at the receiver. Corrections are made accordingly to line up the edges of the data transitions with the local clock. When correspondence is achieved, the local clock is synchronized with the data. The data rate is known and the local clock generates the frequency of the data.
Another known system, somewhat more sophisticated than the one described in the Data Transmission publication is described in the 1981 supplement to the TTL Data Book Second Edition published by Texas Instruments Inc. see pages 38-43.In the TI system, when a data transition or edge is received and recognized, there is maintained an indication of the amount of error. A long up/down counter keeps a running tally of the amount of error. The counter counts up for early edges and down for late edges (or vice versa). The overflow point of the counter is adjustable. When the counter overflows a correction is made. The direction of the overflow determines the direction of the correction.
The above discussed approaches to clock recovery work adequately with many digital data codes. However, these traditional approaches to clock recovery are not suitable for use with Manchester encoded data. Manchester encoded data is a return to zero (RZ) coding scheme rather than a nonreturn to zero (NRZ) code which is suitably decoded with traditional clock recovery arrangements as discussed above. In NRZ code systems, transitions i.e. from "0" to "1" or from "1" to "0" occur at the edges of a bit cell. However, with Manchester coded RZ data, transitions occur in the middle of bit cells. The definition of Manchester coded data is shown in FIG. 2. A logic "1" is characterized by a transition from "0" to "1" in the middle of the data cell. Similarly, a logic "0" is defined by a transition from "1" to "0" in the middle of a data cell. Thus, in Manchester encoded data, transitions can occur both at the edges and in the middle of a data bit cell. This property causes a variety of problems to occur at a receiver that uses edges in the receive stream to lock its local clock. In effect, a traditional clock recovery arrangement would be fooled into thinking that the transition occurring in the middle of a bit cell was actually defining a cell edge. Thus, a more suitable clock recovery arrangement for use with Manchester data is required.